Electron optical system having spiral collimating electrode adjacent the target



Mam}! 1968 K. SCHLESINGER 3, 75,399

ELECTRON OPTICAL SYSTEM HAVING SPIRAL COLLIMATING ELECTRODE ADJACENT THE TARGET Filed Jan. 5, 1966 5 Sheets-Sheet 1 FIG.I.

MODULATING SIGNAL I l i lNVENTO-R: KURT SCHLESINGERQ H58 ATTQRNEYa 3,375,390 TING March 26, 1968 K SCHLESINGER L SYSTEM HAVING SPIRAL COLLIMA ELECTRODE ADJACENT THE TARGET ELECTRON OPTICA 5 Sheets-Sheet 2 Filed Jan. 5, 1966 INVENTOR: KURT .SCJILESINGEI? Mam]! 1968 K. SCHLESINGER 3, 75,390

ELECTRON OPTICAL SYSTEM HAVING SPIRAL COLLIMATING ELECTRODE ADJACENT THE TARGET Filed Jan. 5, 1966 3 Sheets-Sheet (5 INVENTOR: KURT SCHLESINGER United States Patent 0.

3,375,390 ELECTRON OPTICAL SYSTEM HAVING SPIRAL COLLIMATING ELECTRODE ADJACENT THE TARGET Kurt Schlesinger, Fayettevilie, N.Y., assignor to General Electric Company, a corporation of New York Filed Jan. 3, 1966, Ser. No. 518,160 8 Claims. (Cl. 31383) ABSTRACT OF THE DISCLOSURE An electron optical system arranged in coaxial relation along a reference axis comprising an electron beam source, a target spaced from said source, and a nonlinear spiral collimating electrode of increasing electrical resistance per unit axial length adjacent said target.

The present invention relates to ultra high resolution electron optical systems as disclosed in US. Patent 3,223,- 871 Schlesinger assigned to the same assigness as the present invention, and particularly to improved cathode ray tubes which embody such electron optical systems in combination with a specific collimating lens adjacent the target of said tubes.

One object of the invention is to provide an electron optical system for producing an electron beam having a target spot size which is extremely small, e.g., a few microns in diameter, at target beam current levels providing desirable energy transfer to the target, e.g., sev' eral microamperes.

Another object is to provide such an electron optical system wherein the electron beam can be readily defiected through a substantial target-scanning angle, e.g., of the order of 40 degrees, and wherein the overall physical length of the electron optical system is conveniently small.

Another object is to provide an electron optical system of the character described wherein defiection-defoc-using of the electron beam is minimized.

Another object is to provide an electron optical system of the character described including convenient means for precisely controlling the cross sectional shape of the target scanning electron beam.

Another object is to provide an electron optical system of the character described including convenient means for precise-1y controlling the cross sectional shape of the target scanning electron beam.

Another object is to provide an electron optical system of the character described which does not require critical mechanical alignment of the structural elements thereof.

Another object is to provide an improved cathode ray tube embodying an electron optical system of the foregoing character and a non-linear spiral collimating lens adjacent the target of said tube.

These and other objects of the invention will be apparent from the following description and the accompanying drawings wherein:

FIG. 1 is an axial sectional view of a preferred cathode ray tube constructed according to my invention;

FIG. 2 is an enlarged fragmentary view of a portion of the structure of FIG. 1;

FIG. 3 is an enlarged transverse sectional view of the structure of FIG. 1 taken on line 3-3 thereof;

FIG. 4 is an enlarged transverse sectional view of the structure of FIG. 1, taken on line 4-4 thereof;

FIG. 5 is a view similar to FIG. 4 showing an alternative form of one feature of the invention;

FIG. 6 is a schematic diagram of circuitry associated with the apparatus of FIG. 5.

An electronic optical system constructed in accordance 3,375,390 Patented Mar. 26, 1968 with my invention is shown in FIG. 1 embodied in a cathode ray tube which includes an envelope having an elongated neck 2, and an enlarged funnel portion 4 closed by a face plate 6. On-the interior of face plate 6 is a target in the form of a luminescent screen 8. The screen 8 may consist preferably of a continuous transparent film of luminescent material which may be of the vapor-phase deposited type such as taught by US. Patents 2,675,331; 2,685,530 and 2,887,401, commonly assigned herewith.

Arranged in order coaxially within the neck portion of the envelope from the base or rearward end of the neck toward the target screen 8 are a plurality of sections hereinafter to be described in greater detail. These sections include an electron beam generating section 10, a prefocusing lens section 20, an electrically neck-elongating section 30, and a main focus lens section 40.

Referring to FIG. 2, in the electron beam generating section 10 of the tube, electrons are emitted from an axially located cathode 11 which is preferably of the high current-density type such as a dispenser cathode. The emitted electrons pass through axially apertured collimator electrode 12, first anode-13, beam intensity modulating gate electrode 14, and a meniscus electrode 15. As best shown in FIG. 2, anode 13 has a forwardly projecting (towards screen 8) convex surface 16 which, together with the confronting rearwardly directed or facing con cavity 17 of the gate electrode 14 which, as illustrated, is in receiving relationship to the projecting surface 16, form a focusing electrostatic field, the equipotential surfaces of which are hyperboloids of revolution symmetrical with the axis of the tube neck and asymptotic to rear- Wardly concave conical surface 17 of approximately 109 apex angle whose apex faces screen 8 or away from said emitter and lies on the tube neck axis in substantial coincidence with the central aperture 18 in the meniscus electrode. The characteristics of such a focusing electrostatic field are described in more detail in US. Patent 2,995,676 Schlesinger commonly assigned herewith.

The field between the anode 13 and the gate electrode 14 forms in the aperture 18 an effective virtual cathode of demagnified size relative to the actual cathode 11 and thus illuminates the aperture 18 with an electron beam of density many times that of the emission density from the actual cathode 11, for resulting maximum brightness at the screen 8. In a successfully operated tube constructed as herein described, the cathode 11 was operated as ground potential, collimator 12 at about +10 volts, anode 13 and meniscus 15 at about 500 volts, the beam intensity modulation was achieved by varying the potential of gate 14 between 5 and +10 volts, and current densities of about 25 amperes per square centimeter at aperture 18 were achieved with emission density at the cathode of only about 2 amperes per square centimeter.

The electron beam emerges from the aperture 18 into the pre'focusing lens section 20, Within which is formed an electrostatic field whose equipotentials are hyperboloids of revolution coaxial with the tube neck axis, and asymptotic to and within a coaxial forwardly concave conical surface 21 of approximately 109 apex angle having its apex facing emitter 11 and located in substantial coincidence with aperture 18. The forward end of the prefocusing lens section is terminated by a transverse axially apertured conductive wall 22 having a coaxial opening 23 and spaced from the conical surface 21 by a supporting insulating cylinder 24.

The two surfaces 21 and 22 serve to form the hyperbolic electrostatic field within the prefocusing lens section 20, and, if desired, the formation of such field may be augmented by further electrode means, such as resistive coatings on the cylinder 24 having local potentials corresponding to the local space potential of the prefocusing lens field.

In FIG. 1, adjacent transverse wall 22, and closed at its rearward end thereby, is the electrically neck-elongating section 30 which includes an accelerating cylindrical spiral electrode 31 arranged coaxially with the neck axis. The spiral electrode 31 may conveniently consist, as shown, of a conductive spiral coating of uniform pitch on the interior surface of an insulating support cylinder 32. Desirably the spiral electrode 31 has a high impedance of the order of 30-50 megohms to minimize power consumption. Wall 22 and the rearward end of the spiral electrode 31 are connected by a lead 35 to a suitable adjustable potential source, shown schematically as potentiomcter 50, which may provide to lead 35 a relatively low potential of the order of 100 to 800 volts. A conductive coating 33 on the exterior of the insulating cylinder 32 serves as an electrostatic shield for the spiral electrode and also conveniently provides part of a conductive path from the forward end of electrode 31 to a conductor 53 of substantially higher potential, which may be for example of the order of 7,000 volts, so as to provide a substantial accelerating field within spiral electrode 31. Annular conductive caps 27, 36 are provided at each end of the cylinder 32 to facilitate mechanical support and provide convenient electrical connections to the ends of spiral electrode 31.

The forward end of the cylinder 32 is closed by a transverse conductive wall 37 having a central aperture 38. Forward of the aperture 38 and partially supported by wall 37 is the main focusing lens section 40 of the tube, which may be of any suitable type including a unipotential lens, but is here shown as a bipotential lens. Section 40 includes as one element of the bipotential lens a coaxial conductive cylinder 41 of diminished diameter relative to electrode 31 and having an enlarged mouth 42 at its forward end. Cylinder 41 preferably has the same potential as the forward end of spiral electrode 31. The

aperture 38 serves as a limiting aperture minimizing aberration through the lens section 40, and another transverse wall 43 intermediate the ends of cylinder 41 has an axial aperture 44 and serves as a shield to cut down emission of stray electrons from the lens section 40. The forward end of the lens section 40 is spaced by an annular insulator 45 from a supporting conductive sleeve 46 which in turn is connected by conductive support fingers 47 to the neck wall. Sleeve 46 forms the second element of the bipotential lens and is electrically connected by fingers 47 and an internal conductive coating 48 at the front of the neck and on the inner surface of the envelope funnel 4 to the high voltage terminal 51 of the tube, which may have a potential several times that of cylinder 41, for example 20 kv. A suitable deflection yoke 49 is provided for scanning the electron beam on the screen 8.

A preferred structure of this invention is illustrated in FIG. 1 which is a cross sectional view of a preferred embodiment of this invention. In FIG. 1 the continuous coating type electrode 48 has been terminated adjacent the neck section of the bulb 4 to define an electrode 54. Electrode 54 includes a terminal 55 for application of a desired voltage which in one example is about 9.5 kilovolts.

Next adjacent electrode 54 and spaced therefrom toward faceplate 6 is a separate cylindrical band type electrode 56 with an electrical terminal 57. Band electrode 56 is employed as a transitional electrode to stabilize electron trajectories prior to entry into the following electrical field. Cylindrical or band electrodes are employed in conjunction with spiral electrodes, for example between spiral electrodes to smooth out the abrupt changes in electrical fields between spiral electrodes. Next adjacent band electrode '56 and electrically connected thereto is a'spiral electrode 58 similar to the spiral electrode 31 of FIG. 1. Spiral electrode 58 is of the non linear type, i.e., the number of turns or helices per unit axial length of the helix, increases in the axial direction toward target 8. This hon linearity provides for increasing electrical resistance axially and an electrical field pattern which collimates electrons from the main focusing lens to provide more normal landing on target 8. While other forms of electrodes which provide the increased resistance, such as tapered section electrodes, the spiral electrode has been found to be the most feasible. Examples of this and other forms of spiral electrodes are found in US. Patent 3,143,681 Schlesinger, assigned to the same assignee as the present invention. A further description of the spiral electrode '58 found in Correction of Deflection-Aberrations by Analog Computer, Schlesinger & Wagner, IEEE Transactions on Electron Devices, vol. ED-l2, #8, August 1965, pp. 478-484. General information on spiral lenses may be found in Technical Documentary Report No. ASD-TDR- 63-641 Study of Electron Focusing by Non-Linear Spiral, Schlesinger, May 196-3, Project 4156 Task No. 415605, Contract AF33('657)7 682 released to Oflice of Technical Services for unrestricted availability.

A suitable terminal 59 is provided for spiral electrode 58. Suitable potentials at the terminals 57 and 59 have been found to be 6 kv. and 18 kv. respectively. The specific dimensions of the spiral electrode 58 are chosen to provide the desired results in a given tube and application. However, the direct use of an increasing resistance spiral will provide superior performance as compared to electrode 48 of FIG. 1.

In the operation of the electron optical system, an electron beam of high current density, for example of the order of 25 ampes per square centimeter, substantially demagnified in cross section by the focusing field between anode 13 and gate 14, and modulated in intensity by a control signal applied to gate 14, is supplied to aperture 18, and forms there a virtual cathode. The prefocus-ing lens section 20 operates to provide a virtual image of the virtual cathode at aperture 18, which virtual image serves as the object for the main focusing lens 40. It has been found that the accelerating section has the electrical effect of elongating the distance from the principal plane of the main focusing lens to its effective object plane. Thus, since the magnification of the main focusing lens, as is well known to those skilled in the art, is proportional to the image distance divided by the eifective object distance, for a fixed image distance between the screen 8 and the main focusing lens 40 the image size or electron beam spot size on the screen is correspondingly decreased by the neck-elongating or object distance-elongating action of spiral accelerating electrode 31.

Dynamic focusing as well as control of resolution is provided conveniently by varying the potential of the wall 22 relative to the potential of surface 21 of the meniscus electrode 15, which in turn modifies the action of the prefocusing lens section 20. For example, it has been found that as the potential of wall 22 is increased above that of surface 21, the overall effect of the prefocus lens becomes positive or converging, and the virtual image of the aperture 18 produced by the prefocus lens sect1on moves rearwardly and increases in size. Since this virtual image serves as the effective object for the main focusing lens 40, the overall effect at the screen is one of moderate spot size enlargement with a considerable increase in beam current, such that the current density observed at the screen remains substantially constant. When the potential of wall 22 equals that of surface 21 of meniscus electrode 15, the space between meniscus electrode 15 and wall 22 becomes substantially field-free, and the location of the effective object for the main focusing lens is at the aperture 18. When the potential of wall 22 is decrease-d below that of the meniscus electrode, however, it has been found that the overall effectcf the prefocusing lens section 20 is diverging or negative, and the virtual image-of aperture 18 which the prefocus lens forms moves forward of the aperture 18 and is diminished in size. The result of that at screen 8 is that the electron beam spot size gets smaller and beam current decreases somewhat. Thus convenient control of resolution, as well as dynamic focusing, is obtained merely by adjusting the potential of wall 22.

I have found that with a potential at terminal 51 of 20 kv., with cylinder 41 and the forward end of spiral electrode 31 at 7 kv., and with meniscus electrode at 500 volts, in a tube of the type described, excellently small spot sizes of about .00033 inch (8 microns) at screen 8 with 1.5 microampere beam current can be obtained. This spot size can be obtained when the field in the prefocusing lens is decelerating, and the rearward end of spiral electrode 31 and wall 22 have a potential of about 0.6 that of electrode 15, i.e., 300 volts.

Optionally, the neck-elongating section may be arranged to act as a moderate converging lens for the electron beam, as well as an accelerator, for example by changing the spiral 31 from a uniform pitch to one having a pitch progressively increasing toward wall 37. However, in such a case, care must be taken that the converging lens action of the section 30 is not made so strong as to produce a second crossover of the electron beam before it reaches the screen 8, since such a result would cause undesired enlargement of the spot size at the screw.

Since proper centering of the electron beam at aperture 18 and at the limiting aperture 38 at the forward end of the spiral electrode 31 is important to preserve efilcient transmission of beam current through the electron optical system, to alleviate problems of exact coaxial alignment of the various parts of the system as well as to correct for the effect of the earths magnetic field, adjustable centering coils 60 are provided, external to the tube neck in the vicinity of the electron beam generating section 10, to provide centering at aperture 18. A similar set of coils 67 is provided in the vicinity of the spiral electrode 31 to provide centering at aperture 38. To avoid repetition, only coils 60 will be described in detail, it being understood that coils 67 are similar in all respects except for increased length, as is apparent from FIG. 1.

As shown in the sectional view of FIG. 3 the centering coil assembly 60 outside section 10 consists of an insulating cylindrical support 61 on which are mounted two pairs of electromagnetic coils, the windings of the coils being designated AA, BB, CC, and DD for clarity. Coils AA and BB are wound in series and supplied from a remote source of reversible-polarity, adjustable amplitude direct current, which may be for example a battery 62 and a potentiometer 63, and are for centering the beam in the horizontal direction. Coils CC and DD are likewise wound in series and supplied from a remote source of reversible-polarity adjustable amplitude direct current, which as shown may be battery 62 and a potentiometer 64, and are for centering the beam in the vertical direction.

For the purpose of permitting precise Vernier control of the cross sectional shape of the electron beam, in order to enhance spot roundness or if desired to produce spot ellipticity at the screen, an assembly of beam shaping coils 70 is provided external to the tube neck in the vicinity of the main focus lens section 40. This beam shaping coil assembly 70 is shown in detail in the cross sectional view of FIG. 4. It consists of an insulating support cylinder 71 which is rotatably adjustable about the neck axis and on which are mounted four coils EE, FF, GG, and HH each arranged to subtend a 90 angle in a plane transverse to the neck axis. The coils are Wound in series in a sense such as to develop two mutually perpendicular pairs of magnetic poles wherein adjacent poles are of opposite polarity and diametrically opposed poles have the same polarity, as shown in FIG. 4. The coils are connected to a remote source of reversible-polarity adjustable-amplitude direct current, which may be for example a potentiometer 64 and battery 62 as shown in FIG. 3.

Referring to FIG. 4, the N and S negative poles there shown, which result from the current in coils E E, FF, GG, and HH, indicate how the mutually orthogonal forces of an electron beam passing in an axial direction through the field generated by coils EE, FF, GG and HH tend to correct ellipticity of the beam. A beam having an undesired degree of ellipticity, for example with a major axis 95, can be conveniently rendered circular in cross section merely by properly rotating the coil assembly 70 so that the beam compressing transverse magnetic fields thereof, as shown by poles N and S and vectors 72, coincide with and compress .the major axis of the ellipse, and the beam expanding transverse magnetic forces thereof, as shown by vectors 73, coincide with and expand the minor axis of the ellipse, and adjusting the current through the coils of assembly 70 for the desired degree of roundness of the beam. Conversely, if for any reason a desired degree of ellipticity is required, such may be obtained even from a beam of perfectly circular cross section by suitably angularly adjusting the coil assembly and properly adjusting the amount of current through the coils of assembly 70.

It will thus be appreciated from FIG. 4 that the coil assembly 70 provides magnetic fields through the tube neck at the main focus lens section 40 which cannot provide any deflection of the beam in a manner such as would afiect its centering but which do distort or change as desired the cross sectional shape of the electron beam.

Alternative means for Vernier control of the cross sectional shape of the electron beam are shown are in FIGS. 5 and 6. The apparatus of FIGS. 5 and 6 has the advantage that it effects control of beam shape entirely electrically, eliminating any need for mechanical rotation of a coil assembly around the tube neck. This alternative beam shaping means includes an assembly of coils arranged external to the tube neck in the vicinity of the main focus lens section 40, and shown in detail in FIG. 5. The coils of FIG. 5 are arranged and energized to provide magnetic fields which produce pairs of force vectors which act along the major and minor ellipse axes of the electron beam, similar to the action of vectors 72 and 73 of FIG. 4, except that with the apparatus of FIGS. 5 and 6 the force vectors can be electrically rotated through 360 to any desired angle of orientation, and the amplitude of each pair of vectors can be electrically varied from a maximum in one sense through zero to a maximum in the opposite sense. Thus the apparatus of FIGS. 5 and 6 can correct for, or introduce, any degree of ellipticity of the beam at any angular orientation of the ellipse axes.

Turning to FIG. 5, eight identical coils are provided in two sets of four coils each, designated A1, A2, A3, A4, and B1, B2, B3, B4. The coils of the A set subtend adjacent angles of at the neck axis, and the coils of the B set likewise subtend adjacent angles of 90 but are displaced 45 relative to the A coils. Each of the two current paths of each coil which extends parallel to the neck axis is centered in a sector subtending an angle of 22 /2 at the neck axis. Designating the current flow directions by plus and minus signs, the coil arrangement will be evident from FIG. 5. All four A coils are wound in series and are supplied by direct current of controllable amplitude and reversible polarity, and all four B coils are Wound in series and supplies by direct current of controllable amplitude and reversible polarity, separately from the A coils. The diametrically spaced coils of each set, e.g., coils A2 and A4, provide magnetic poles of one polarity, say the North poles designated NA in FIG. 5, while the remaining coils A1 and A3 of the same set provide poles of opposite polarity, shown as SA in FIG. 5. Such poles NA and SA correspond in function and effect to the N and S poles of FIG. 4. Likewise the B coils provide poles SB and NB of FIG. 5. Thus the eight coils are capable of providing eight magnetic poles spaced 45 about the axis of the electron beam. The polarity of any diametrically spaced pair of poles is always identical, and the polarity of any 90 spaced poles is always different, so that regardless of the effect on ellipticity there is no net effect on the centering of the electron beam. Moreover, by the circuit of FIG. 6 hereafter to 'be described, the strength and polarity of each such pole may be adjusted from maximum in one sense through zero to maximum in the opposite sense, and thus the summed values of the mutually perpendicular pairs of force vectors exerted on the electron beam by such poles may be madeto rotate to any angle and have any desired amplitude.

The circuit of FIG. 6 includes a pair of field intensity control potentiometers 105, 113, which provide for control of the amplitude of the current fed to the A and B coils, respectively, and a pair of field orientation control potentiometers 103, 111. As will be explained in detail hereinafter, the potentiometers 103 and 111 serve to impose a sine-cosine relationship on the variations in current amplitude and polarity supplied the respective A and B coil sets, and this enables control of the angular orientation of the summed force vectors produced by the magnetic poles of FIG. 5, throughout the entire 360.

As shown in FIG. 6, one side of the A coil series is grounded, and the other side is connected through a current limiting resistor 101 and field orientation control potentiometer 103 to the field intensity control potentiometer 105 which with battery 107 provides a source of reversible polarity adjustable amplitude direct current. Likewise one side of the B coil series is grounded and the other side is connected through resistor 109 and orientation control potentiometer 111 to a source of reversible polarity adjustable amplitude direct current at intensity control potentiometer 113. The movable contacts 140, 141 of potentiometers 105, 113, are ganged, and their stationary windings are connected to battery 107 with opposite polarity.

Potentiometer 103 has four equal segments separating five terminals 115, 123, 124, 125, 126, and potentiometer 111 likewise has four equal segments separating five terminals 116, 127, 128, 129, 130. The movable contacts 135, 136 of potentiometers 103 and 111 are ganged.

The terminals 115, 124 and 126 of potentiometer 103 and the terminals 128 and 130 of potentiometer 111 are grounded, while terminals 123 and 129 are joined, 125 and 127 are joined, and 116 and 126 are joined.

Intensity control potentiometer 113 determines the maximum current of one polarity available to the B coils from the terminal 129 of potentiometer 111 and available to the A coils from terminal 123 of potentiometer 103, while potentiometer 105 controls the maximum current of the opposite polarity available to the A coils from terminal 125 of potentiometer 103 and available to the B coils from terminal 127 of potentiometer 111.

As the movable contacts 135, 136 of potentiometers 103 and 111 are moved the currents to the A and B coil sets available from contacts 135, 136 vary from zero to a maximum in one sense, through zero again and to a maximum of the opposite sense with a 90 phase difference and hence have a continuous relative approximately sine-cosine relationship. These sine-cosine related currents produce vector summed magnetic fields which can be rotated to any angular orientation in FIG. 5, and thus the setting of the movable contacts of potentiometers 103 and 111 enables the summed force vectors affecting the ellipticity of the electron beam to be rotatively oriented at any angle throughout the 360 range. Moreover the settings of potentiometers 103, 113 control the maximum values of the currents available from contacts 135, 136, and hence control the amplitude of the force vectors affecting the ellipticity of the electron beam. Thus merely by adjusting the pairs of movable contacts 140, 141, and 135, 136, any degree of ellipticity may be introduced into or removed from the electron beam, without affecting its centering.

Thus there has been shown and described a high resolution electron optical system for a cathode ray tube or the ilke which permits the generation of extremely fine electron beam spot sizes of as little as a few microns diameter and within an overall length which is conveniently small. The mechanical structure of a cathode ray tube embodying suc-h electron optical system is relatively simple and rugged, and associated means surrounding the axis of the system is provided for obtaining precise centering of the electron 'beam and thereby relaxing the tolerances of alignment of the parts. Additionally precise vernier control of the beam cross sectional shape is provided so that substan ial scanning angles of the order of 40 can be obtained with minimum defocusin A cathode ray tube constructed as above described has been found to provide very large screen current density of the order up to 2.0 amperes per square centimeter, with screen power loadings of up to approximately 50 kw. per square centimeter at 20 kw. screen potential and with a resolution of up to the equivalent of 10,000 raster lines on a 5" tube face. Such resolution is the equivalent of up to 400 complete TV pictures arranged in a square or up to million dots or 25 million bits of four increments per bit.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than those illustrative embodiments heretofore described. Accordingly it is to be understood that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron optical system comprising, arranged in coaxial relation along a reference axis, an electron beam source, a target spaced from said source, a main electron beam focusing lens disposed between said source and said target, prefocusing electron lens means between said electron beam source and said main lens for providing an electron optical virtual image of said electron beam source for imaging on said target by said main focusing lens, a pair of electrode means in said prefocusing electron lens means and arranged for the application of different potentials thereto for adjusting the axial position of said virtual image, and a collimating electrode means between said main lens and said target to effectuate normal landing of electrons of said target, said collimating electrode being a non-linear spiral electrode of increasing electrical resistance per unit axial length towards said target including a cyclindrical constant electrical resistance portion at the end thereof facing said electron beam source.

2. In an electron optical system comprising, arranged in coaxial relation, an electron beam source, a target spaced from said source, a main electron beam focusing lens disposed between said source and said target, an electron beam prefocusing electron lens disposed between said electron beam source and said target for providing an electron optical image of said electron beam source for imaging on said target by said main focusing lens, and accelerating spiral electrode means providing an axial electron accelerating field between said prefocusing lens and said main lens, and means for varying the strength of said accelerating field to vary the effective object distance of said main lens to reduce spot size on said target, the improvement comprising a spiral collimating electrode of increasing resistance towards said target coaxially positioned between said main focusing lens and said target to provide normal landing of electrons on said target.

3. The invention as recited in claim 2 wherein said spiral collimating electrode is a non linear spiral having increasing electrical resistance per unit of axial length in a direction towards said target.

4. In an electron optical system comprising, arranged in coaxial relation along a reference axis, an electron beam source including an emitter, electrode means forming an electrostatic field having equipotential surfaces which are hyperboloids asymptotic to a conical surface of approximately 109 apex angle coaxial with said reference axis and with said apex facing away from said emitter, a target spaced from said source, a main electron beam focusing lens disposed between said source and said target, spiral accelerating electrode means providing an axial electron accelerating field between said main focusing lens and said electron beam source, and a prefocusing electron lens between said source and said accelerating electrode means for providing an electron optical virtual image of said electron beam source for imaging on said target by said main focusing lens, the improvement comprising a non linear spiral electrode positioned coaxially between said main lens and said target and having increasing electrical resistance in the axial direction towards said target to provide normal landing of electrons on said target.

5. In an electron optical system comprising, arranged in coaxial relation, an electron beam source, a target spaced from said source, a main electron beam focusing lens disposed between said source and said target, prefocusing electron lens means between said source and said main lens for providing electron optical virtual image of said electron beam source for imaging on said target by said main lens, electrical means for controlling the effective object distance of said main lens, electron beam ellipticity control means including electromagnetic coil means for forming a plurality of transverse magnetic fields in the path of said beam and having diametrically spaced pairs of magnetic poles quadrilaterally spaced in a plane transverse to said beam, the poles of each diametrically spaced pair being of the same polarity and the adjacent poles of each quadrilaterally spaced set being of opposite polarity, and means for varying the strength and polarity of said poles, the improvement comprising (a) a non linear spiral collimating electrode positioned coaxially between said main lens and said target,

(b) said spiral lens having increasing electrical resistance in a direction toward said target to provide normal landing of electrons on said target.

6. In an electron optical system including an electron beam source and a target spaced from said source along a reference axis, said electron beam source including an anode electrode and a gate electrode, said anode electrode having a forwardly projecting convex surface coaxial with said reference axis, said gate electrode having a rearwardly directed concavity coaxial with said axis, said concavity being centrally apertured for passage of said electron beam, said gate electrode being adapted to have applied thereto modulation signals for varying the flow of electrons in said beam, prefocus lens means coaxially disposed between said target and said beam source and including an electrode having a forwardly concave conical surface of approximately 109 apex angle caxial with said axis and with the apex thereof facing said electron beam source, an axially apertured transverse electrode axially spaced forward of said conical surface, means forming an axial virtual cathode opening in said conical surface, means for directing said electron beam axially through said virtual cathode opening, electrode means forming a coaxial main focus lens intermediate said prefocus lens and said target, a coaxial cylindrical spiral electrode extending between said prefocus elens and said main focus lens for forming a uniform axial accelerating field, the improvement comprising (a) a non linear spiral collimating electrode coaxially positioned between said main lens and said target,

(b) said spiral having increasing resistance per unit axial length in a direction towards said target,

(0) and a cylindrical band electrode coaxially positioned between said spiral electrode and said main focusing electrode and electrically connected to said spiral electrode.

7. In an electron optical system including an electron beam source and a target spaced from said source along a reference axis, said electron beam source including an anode electrode and a gate electrode, said anode electrode having a forwardly projecting convex surface coaxial with said reference axis, said gate electrode having a rearwardly directed concavity coaxial with said axis, said concavity being centrally apertured for passage of said electron beam, said gate electrode being adapted to have applied thereto modulation signals for varying the flow of electrons in said beam, prefocus lens means forming between said target and said beam source an electrostatic field having equipotential surfaces which are hyperboloids asymptotic to a forwardly concave conical surface of approximately 109 apex angle coaxial with said reference axis with the apex of said conical surface facing said electron beam source, means for directing said electron beam axially into said prefocus lens field, electrode means forming a coaxial main focus lens intermediate said prefocus lens and said target, axially extending cylindrical electrode means forming a uniform axially extending electron accelerating field between said prefocus lens and said main focus lens and said axially extending cylindrical electrode means having a coaxially apertured transverse wall portion at each end thereof, the improvement comprising (a) a non linear spiral collimating electrode coaxial positioned between said main lens and said target,

(b) said spiral having increasing resistance per unit axial length in a direction towards said target,

(c) and a cylindrical band electrode coaxially positioned between said spiral electrode and said main focusing electrode and electrically connected to said spiral electrode.

8. In an electron optical system including a reference electron beam axis, coaxial prefocus lens means forming an electrostatic field having equipotential surfaces which are hyperboloids asymptotic to a forwardly concave conical surface of approximately 109 apex angle coaxial with said axis, coaxial cylindrical spiral electrode means extending axially from said prefocus lens from the larger opening of said conical surface and forming a uniform axial electron accelerating field, and an axially apertured transverse electrode electrically connected to said spiral electrode separating said prefocus lens and said spiral electrode, and means for adjusting the potential of said transverse electrode, the improvement comprising (a) a non linear spiral collimating electrode positioned coaxially with and axially spaced from said transverse electrode to have a different potential applied thereto,

(b) said collimating electrode having increasing electrical resistance per unit axial length in a direction away from said transverse electrode to collimate electrons emerging therefrom.

JAMES W. LAWRENCE, Primary Examiner. VINCENT LAFRANCHI, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5,375,390 March 26, 1968 Kurt Schlesinger It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 22, for "assigness" read assignee lines 45 to 48, strike out "Another object is to provide an electron optical system of the character described including convenient means for precisely controlling the cross sectional shape of the target scanning electron beam."; column 2, line 45, for "as" read at column 4, line 14, for "63-641" read 63:461 column 6, line 63, for "supplies" read supplled column 8 line 46 for "of", second occurrence read on column 10,'line 33, for "coaxial" read coaxially (S Signed and sealed this 22nd day of July 1969 Attest:

Edward M. Fletcher, Jr. E. SCHUYLER, JR-

Attesting Officer Commissioner of Patents 

